1. Field of the Invention
The invention relates to terahertz generation systems and more particular to terahertz generation systems having a pump-probe configuration.
2. Description of the Known Art
Time-domain terahertz generation systems employ terahertz generation and receiver devices which are triggered and powered by laser pulses of sub 300 femtosecond durations. The terahertz devices generate and/or detect electromagnetic pulses or pulse sequences with a Fourier transform possess spectral content in the 0.020 to >10 terahertz range. Most commonly these terahertz electromagnetic pulses are near single cycle sub-picosecond transients in the 0.050 to 4 terahertz range.
There are several common terahertz transmitter and receiver devices which are used in a pump-probe configuration. These include non-linear electro-optic crystals (such as ZnTe, GaP and others) and photoconductive semiconductor devices (“Auston Switches,” fabricated on GaAs, InGaAs, and other ultra high-speed semiconductors). A chief difference among these is the center wavelength of the optical drive in the pump-probe apparatus (GaAs approx. 800 nm and InGaAs between 1000 and 1600 nm, for example). For illustration purposes, photoconductive devices will be discussed. In a photoconductive terahertz transmitter, a bias photoconductive switch gap is illuminated by the femtosecond laser pulse. Carriers are generated and a current flows across the bias gap, quickly rising with the temporal profile of the excitation laser pulse, and quickly falling with the recombination of the carriers. This time varying current creates the terahertz pulse through Maxwell's equations.
The generated terahertz pulse electromagnetic field is detected by a method of highly precise time gating, usually referred to as a pump-probe method. The first laser pulse which drives the terahertz transmitter is the pump. A second laser pulse, precisely time delayed with femtosecond or sub-femtosecond precision with respect to the first terahertz transmitter pulse is the probe, as illustrated in U.S. Pat. Nos. 6,320,191 and 6,849,852; the entirety of both is hereby incorporated by reference. The probe pulse illuminates an un-biased photoconductive switch. The switch can carry current for sub-picosecond duration. If the femtosecond laser pulse driving the terahertz receiver is time coincident with a portion of the generated terahertz pulse electromagnetic field, current will flow across the switch. The current is proportional to the amplitude and sign of the terahertz pulse field at that particular time. Typically, the timing between the pump and the probe is systematically varied and the time-domain terahertz waveform is recorded as a function of terahertz receiver current vs. the delay between the pump and the probe.
Regardless of the terahertz transmitter or receiver device employed in the pump-probe apparatus, the optical path length of the pump beams and probe beams must be known and kept stable to femtosecond or sub-femtosecond precision. If the path length is not stable, this can introduce short term jitter or long term timing drift. This timing error will distort the measured terahertz wave form in a number of ways. For example, the Fourier transform and a time domain terahertz spectroscopy measurement will have power distributed to incorrect frequencies. In another example, the jitter and drift will distort the time of flight of the terahertz pulse through materials such as paper, resulting in incorrect thickness measurement derived from the terahertz post timing.
In previous implementations, time domain terahertz instrumentation would deliver the pump and probe beams entirely in a free space environment. This free space environment's stability is entirely dependent on the mechanical stability of the optical transport. However, because of this dependence, free space transport is a highly restrictive implementation. The femtosecond laser providing the optical pump and probe, as well as the optical delay mechanism must be permanently attached to the same mechanical structure holding the terahertz transmitter and receiver devices. These terahertz transmitter and receiver devices are not freely positionable with respect to each other and the other components of the system without disassembling the system and then reassembling the entire system in a new position.
Freely positionable terahertz transmitter and receiver modules can be constructed by delivering the pump and probe femtosecond optical drive through optical fibers. The optical fibers are typically single mode, or single mode polarization maintaining. The fibers allow the terahertz transmitter and receiver modules to be moved together or independently during instrument configuration or while the instrument is in use (for example, the transmitter and receiver can be mounted in a raster scan imaging gantry—this allows the object under test to remain stationary; a free space system usually would require the object to move while the free space terahertz system remains stationary). The fibers guide the pump and probe optical drive as the fiber bends, as well as mostly, but often times insufficiently maintain a constant optical path length in both the pump and probe optical path. As long as the motion of the fibers consists of simple bending which does not stretch or put the fibers under tension, the optical path length remains constant. Unfortunately, it is possible that motion of the optical fibers during the pump and probe procedure can be inadvertently stretched during motion. This can occur by simple tension or through the bending and other motions which cause the optical fibers to bind within the cable jacket, bulkhead, or other locations which induces tension and stretching.
The pump and/or probe pulse group travel time through the optical fibers will increase if the optical fibers are stretched and/or tensioned. This group delay results through one or more physical effects, chiefly increase in the length dimension of the fiber core, and/or strain induced changes in the group velocity, which is related by the first derivative of the index of refraction verses frequency. These unintentional delays of several picoseconds which may result from stretching or tensioning the optical fibers may severely distort the time domain terahertz waveform generated and/or detected by the pump-probe method.